The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
In some control systems it is desirable to measure the frequency of an oscillator. The oscillator may be employed to provide high-frequency energy to an inductive heater. An example of such an application may be found in engine control modules for fuel-injected internal combustion engines. In those applications the oscillator powers an inductive heater that is incorporated into the fuel injector. The inductive heater heats the fuel during predetermined conditions, such as cold starts, to improve fuel atomization from the injector. The improved fuel atomization helps to reduce hydrocarbon emissions and improves cold start performance, particularly when using fuel that includes ethanol. Ethanol becomes increasing difficult to atomize as it gets colder.
It is undesirable to measure the oscillator frequency by directly measuring its output current or voltage. The output current and/or voltage may be hundreds or thousands of times higher than other signals in the engine control circuitry. Directly measuring the current is additionally undesirable as it may introduce a loss component such as a sense resistor or the wire of a Hall sensor. These components also add cost to a measurement circuit.
Measuring the oscillator frequency can be a common-mode and measurement problem which can be solved with higher cost components. Typical devices for frequency-to-voltage conversion are either too expensive for automotive serial production, such as the Analog Devices AD650, or not readily available, or require somewhat complex additional circuitry such as a charge-pump or a PLL (Phase-Locked Loop) integrated circuit.